1. Field of the Invention
The present invention refers to an apparatus for treating the surface of workpieces by means of a plasma torch. Such treatment is performed in an inert operating gas atmosphere and the workpieces to be treated are of a substantially thermically sensitive material. The apparatus of the invention comprises, according to the prior art, a work station having a plasma gun member and means for receiving a workpiece to be treated. A gas-tight container is provided enclosing said work station. Further, there are provided cooling means for cooling the surface of the workpiece to be treated. The cooling means includes a fluid cooling medium directed against the surface of the work piece to be treated.
Surfaces of workpieces, even surfaces of workpieces consisting of a thermo-sensitive material, as for example plastic materials, can be treated by means of a plasma torch. Such treatment may include providing a coating on the surface of the workpiece or preparing for such coating. However, it is essential that the surface portion of the workpiece which just has been treated and, thereby, has been heated up by the plasma torch, is cooled immediately after treatment by the plasma torch, e.g. by a stream of cooling gas directed towards the surface of the workpiece. Thus, it is possible to keep the surface temperature of the workpiece within safe limits with the result that the material structure of the workpiece is not altered and the workpiece will not be destroyed.
2. Prior Art
U.S. Pat. No. 4,675,205 discloses a method and an apparatus suitable to treat workpieces which are symmetric with regard to their central axis. This apparatus comprises a movable plasma gun producing a plasma stream directed to the surface of the workpiece to be treated. In order to cool the workpiece surface just treated by the plasma gun, a cooling member is provided which generates a cooling medium stream directed to the surface of the workpiece. This cooling member is movably mounted as well whereby the movements of the plasma gun and the cooling member are coordinated such that the operation area on the surface of the workpiece of the cooling medium stream immediately follows the operation area of the plasma torch on the surface of the workpiece.
As the region of movement of the plasma gun as well as of the cooling member is practically limited to a predetermined extent due to the connecting pipes and cables leading to the plasma gun and the cooling member, respectively, the workpiece to be treated is rotated while the path of movement of the plasma gun and the cooling member, respectively, runs only in a plane containing the rotation axis of the workpiece.
In this apparatus of the prior art, a cooling liquid is provided which has a temperature which is near its saturation temperature. Preferably, a liquefied gas, e.g. helium, nitrogen or argon is used as cooling liquid.
Practice has shown that in an apparatus according to the disclosure of U.S. Pat. No. 4,675,205 that a considerable throughput of cooling medium is required for an effective cooling of the workpiece to be treated, even if the temperature of the liquefied gas is extremely low and even if the cooling is performed only within a comparatively small surface area of the workpiece. Thereby, the movable cooling member comprises only one single output nozzle for the liquefied gas. The disadvantage of this design is that a very high consumption of the expensive cooling medium results and that the constructive measures for the control of the movement of the cooling member, i.e. such a control that it always follows the path of the plasma gun, are very demanding; thus, the costs for the operation of such an apparatus and the manufacturing costs of a cooling apparatus of this kind are very high.